Quadratrix of Hippias, now a Good Article on Wikipedia: https://en.wikipedia.org/wiki/Quadratrix_of_Hippias

This is a curve swept out by the crossing point of two moving lines, one rotating and one translating. The ancient Greeks discovered that if you have a copy of this curve already, you can use it to trisect angles and construct a square with the area of a given circle, two classical geometry problems that are impossible with compass and straightedge alone. As I wrote on an earlier blog post, https://11011110.github.io/blog/2025/03/22/ancient-greek-mathematics.html, you can also see curves like this when you take a photo of a spinning airplane propellor using a camera with a rolling shutter: